OSB is made from flakes that are created from debarked round logs by placing the edge of a cutting knife parallel to a length of the log and slicing thin flakes from the log. The thickness of the flake is approximately 0.010 to 0.030 inch. The cut flakes are subjected to forces that break the flakes into strands having a length parallel to the grain of the wood several times the width of the strand. The strands can be oriented on the board forming machine with the strands predominantly oriented in a single, e.g., cross machine direction in one layer and predominantly oriented in the generally perpendicular (machine) direction in adjacent layers. The various layers are bonded together by natural or synthetic resin(s) under heat and pressure to make the finished OSB product.
The common grade of OSB is used for sheathing walls and decking roofs and floors where strength, light weight, ease of nailing, and dimensional stability under varying moisture conditions are the most important attributes. In these applications, the appearance and/or weathering of the rough surfaces are not of concern since the product will be covered with roofing, siding, or flooring. Because of the unfinished attributes of utility grade OSB, it commands a relatively low price in the marketplace and is sold at a discount to structural grades of softwood plywood.
The light weight, ease of nailing, and dimensional stability of OSB are attributes much desired in siding products but, due to the irregular surface, OSB has required surface modification before being used as siding or otherwise where aesthetics is important to the consumer. If the material could be imparted with the surface smoothness, coatability, and weatherability of hardboard while retaining its other desirable structural properties, it would be significantly improved in comparison to the commodity structural grade. Others have pursued this objective along different lines with partial success.
One attempt to prevent telegraphing is described in Greten U.S. Pat. No. 3,098,781. The Greten '781 patent discloses a particle board product made from materials, such as flakes, wherein the flakes are graduated in size from the center or core to the outer surfaces, with the coarest flakes at the core and the finer flakes, together with fines, at one or both outer surfaces. The Greten produced particle board is disclosed to have the advantage of accepting an overlay of veneer, paper or plastic sheets without "telegraphing" the relatively irregular surface of the underlying particle board.
Similar OSB siding products are commercially sold that include a resin-bonded overlay of paper laminated to one surface. The paper can accept a limited degree of embossing but it cannot stretch to accept deep, embossing. When embossed beyond a certain depth, the paper ruptures from the tensile strain and reveals the underlying flakes. Furthermore, exposure to the weather causes irreversible swelling of the flakes in thickness which telegraphs the structure of the underlying baseboard (OSB) through the thin overlay and creates a bumpy, irregular exposed surface. The result is an unsightly appearance of the front surface, especially of product that is unembossed or only slightly embossed.
Another example is described in Wentworth U.S. Pat. No. 4,364,984 where wood fines are distributed on the surface of the flake baseboard (OSB) graduated with the coarest wood fines adjacent to the flakes and the finest on the visible surface. Since the fines are bundles of wood fibers which retain the stiffness of wood, they do not consolidate into a tight surface, but rather, retain susceptibility to the ready entry of water and do not holdout paint to a satisfactory degree.
Similarly, Ufermann, et al. U.S. Pat. No. 4,068,991 discloses a particle board, e.g., chipboard product that includes a continuous particle size gradient between a coarser particle core and a finer particle surface layer wherein the particle size gradient transition from one particle size to another can be continuous or step-wise.
Others have disclosed the manufacture of laminates of plywood or particle board with a wet process fiberboard surface, e.g., Birmingham U.S. Pat. No. 2,343,740; Bryant U.S. Pat. No. 3,308,013 and Shaner, et al. U.S. Pat. No. 4,361,612 discloses forming an oriented strand board (OSB), that may be in three or more layers, formed from a mixture of hardwood species and then laminating the OSB to a veneer, wet process hardboard or plywood face panel.
One of the problems associated with the application of an overlay onto an OSB baseboard is that of achieving a strong bond at the interface between the OSB and the overlay capable of resisting weathering. The above-described Wentworth U.S. Pat. No. 4,364,984 suggests that a strong bond can be achieved at the interface between an OSB product and a fine particle overlay by manufacturing the OSB with the largest OSB flakes at the interface, and applying the overlay fine particles such that the longest fines are disposed at the interface. Similarly, the Shaner, et al. U.S. Pat. No. 4,361,612 discloses that shorter fibers in the surface of an OSB product will degrade the bending strength of an OSB product. Further, the Shaner '612 patent teaches that a laminated wood product that includes a flakeboard core laminated to a wood veneer, a wet process hardboard or a wet process fiberboard overlay, as in typical plywood practice, may need a core finishing operation on a drum sander to achieve a core surface capable of good bonding to the overlay.
Bryant U.S. Pat. No. 3,308,013 suggests that a water laid fiber sheet containing resin and having a basis weight of dry fiber from 30 to 750 pounds per thousand square feet can be employed to mask defects in plywood, particleboard, and the like. These heavy papers have been used to produce medium density overlain plywood that has found application in road signs where the smooth surface accepts lettering and reflective laminates. High cost, limited embossability, poor weathering, and poor adhesion of coatings preclude the use of this product in siding applications.
It has been found, in accordance with the present invention, that to achieve excellent embossing fidelity (the capability of achieving a sharp, accurate and permanent transference of an embossing plate design from an embossing plate to a board surface) in an OSB fiberboard overlay, the fiberboard overlay should be air laid (formed by the dry process) onto the OSB surface. If the fiberboard overlay applied over an OSB surface is water laid (formed by the wet process), as suggested in the prior art, the sharp corners and other embossing precision necessary for high quality transference of an embossing plate design is not possible. Unexpectedly, it has been found that the application of a dry layer of a mixture of defibrated fiber and resin binder over an OSB surface enables exact and precise transference of embossing plate details into the surface of the fiberboard overlay. Further, the bonding achieved at the interface between the OSB and the dry process fiberboard overlay, and the resistance to weathering of the fiberboard overlay are unexpectedly better when the fiberboard overlay is formed by the dry process (the fibers are laid over the OSB by gravity from a mixture with air, or mechanically, and are contacted with a binder resin during the fall of fibers onto the OSB surface, and generally contain less than about 15% water). As set forth in more detail hereinafter, the bonding is unexpectedly higher and the boil swell values substantially lower for the OSB-fiberboard composite products of the present invention than for a similar product that includes a fiberboard overlay applied by the typical wet process.
It has heretofore been generally accepted by those skilled in the art that an OSB baseboard and a fiberboard overlay will not form a good bond at their interface and that the differential in dimensional and elastic properties of the two materials will result in delamination because of moisture cycling due to weather conditions. This conventional wisdom also advised against using dried board trim waste as a raw feed to the fiber pulping operation because of residual bonded and consolidated resin. While this theory has been verified for OSB-wet process fiberboard composite structures, surprisingly and unexpectedly, excellent bonding and resistance to weathering is achieved in accordance with the present invention by applying a fiberboard overlay by the dry process to an OSB baseboard. Additional advantages are achieved by forming the OSB such that the smallest flakes of the OSB are disposed at the fiberboard interface, as will be described in more detail hereinafter. Furthermore, those skilled in the art have anticipated warping of the product if the overlay was applied only to one surface but, in accordance with the present invention, it has been found that the expected warping does not occur when the fiberboard overlay, applied to only one major surface, is applied by the dry process.